1. Field of the Invention
The present invention relates to a method of removing iodine compounds from crude acetic anhydride or a mixture of crude acetic anhydride and crude acetic acid, i.e., a carbonylation reaction product.
2. Description of the Related Art
Acetic anhydride is used on a large scale as a starting material for cellulose acetate, and it is useful also as a starting material for chemicals such as medicines, perfumes and dyes. Acetic acid, which is also used on a large scale as a starting material for acetic esters, acetic anhydride, vinyl acetate and terephthalic acid, is a fundamental compound required in the polymer industry, the chemical industry and many other industries.
Acetic anhydride was produced from a ketene obtained by thermal decomposition of acetic acid in the prior art. Recently a process for producing acetic anhydride from methyl acetate or dimethyl ether, and carbon monoxide has been developed. This process comprises conducting a catalytic reaction in the presence of a rhodium compound and methyl iodide as the main catalysts. Various cocatalysts have been proposed, however, since the reaction velocity is low when only the main catalysts are used. The function required of The cocatalyst is to accelerate the reaction of methyl acetate or dimethyl ether with carbon monoxide to an industrially acceptable extent. It is also required of the cocatalyst to exhibit a remarkable acceleration effect in as small as possible amount. When no cocatalyst is used, the reaction velocity does not depend on the concentration of the rhodium compound and methyl iodide used as the main catalysts but it heavily depends on the concentration of methyl acetate. Thus, the productivity of the reaction is very poor. On the other hand, when the cocatalyst is used, the dependence of the reaction velocity on the concentration of methyl acetate is reduced and the reaction velocity comes to depend on the concentration of the rhodium compound and methyl iodide used as the main catalysts.
As the reaction promoters, various cocatalysts are proposed such as iodides of quaternized amine compounds and phosphine compounds, lithium compounds such as lithium iodide and lithium acetate and Lewis acid compounds such as aluminum compounds and chromium compounds. Among these cocatalysts, the iodides of quaternized amine compounds and phosphine compounds have a relatively poor reaction-promoting effect and, therefore, they must be used in a large amount. Lithium compounds such as lithium iodide and lithium acetate must be used also in an amount of as large as 30 to 50 mol or above per mole of rhodium. Although Lewis acid compounds such as aluminum compounds, chromium compounds and zirconium compounds exhibit their excellent effect in an amount of as small as about 10 mol per mole of rhodium, it is necessary that an alkali metal iodide or the like is further added to the reaction system in order to stabilize rhodium and the cocatalyst per se.
Although the reactivity is improved by using a large amount of such a cocatalyst containing iodide ions or a Lewis acid compound together with an alkali metal iodide, the iodide ion concentration in the reaction liquid is increased to increase the concentration of ionizable iodine compounds (i.e. an iodine compound which releases iodide ions upon reaction with water), i.e. the concentration of iodine compounds such as acetyl iodide, hydrogen iodide and inorganic iodine salts in the reaction liquid. When the concentration of such ionizable iodine compounds in the reaction liquid is high, the amount of iodine compounds, other than methyl iodide used as the cocatalyst, such as acetyl iodide and hydrogen iodide as contaminants is increased in the reaction mixture comprising the intended acetic anhydride obtained in the form of vapor. As a result, much labor is required for removing iodine contaminant from crude acetic anhydride or from a mixture of crude acetic anhydride and crude acetic acid and, in addition, another problem of corrosion of the apparatus is caused in the step of purifying the product.
Various processes have hitherto been proposed for removing iodine compounds from a carbonylation reaction product. For example, a process wherein crude carboxylic acid is distilled in the presence of an alcohol to obtain highly purified carboxylic acid; and a process wherein a methanol stream is introduced into a lower part of a distillation column to obtain acetic acid substantially free from hydrogen iodide and methyl iodide [see U.S. Pat. No. 4,039,395 (patented on Aug. 2, 1977; assignee: MONSANTO CO.)] have been proposed. As the result of the investigations made by the present inventors, it was found that although the iodine concentration was lowered to a practical level after the treatment when the liquid to be treated had a low iodine concentration, the complete removal of iodine from the liquid to be treated in the distillation column was difficult when the liquid to be treated had a high iodine concentration in these processes.
A process, wherein a carbonylation product is treated with peracetic acid and diacetyl peroxide and then the product is separated by distillation to reduce the total amount of iodine in the liquid after treatment to 20 ppb or below, has been disclosed. However, also in this process, the total iodine amount in the carbonylation product to be treated is limited to 100 ppm or below, and in addition, the handling of remaining unreacted peracetic acid and diacetyl peroxide is troublesome, since they might cause an explosion by violent reaction thereof. Thus, in practice, this technique cannot be easily applied to the plant unfavorably.
Japanese Patent Publication-A No. 58-118438 (published on Jul. 11, 1983) proposes a process wherein an alkali metal and/or an alkali metal oxide, hydroxide, carbonate or hydrogencarbonate is introduced into acetic anhydride in the distillation step. Although this process is effective in removing iodides having a concentration of not above 100 ppm, a large amount of the alkali metal and/or the alkali metal oxide, hydroxide, carbonate or hydrogencarbonate is necessitated when the iodide concentration in the liquid to be treated is high.
Japanese Patent Publication-B No. 61-8811 (published on Mar. 18, 1986) discloses a process for purifying crude acetic acid by adding methyl acetate to the crude acetic acid containing 1 to 5,000 ppm (determined as the amount of iodine) of iodine compounds in the distillation column before distillation. Although the iodine concentration in the resulting acetic acid is reduced to 0.1 ppm by the purification when the concentration of the iodine compounds in the crude acetic acid to be treated is as low as 20 ppm as described in the Examples of the above patent document, this process also has a defect that the iodine concentration cannot be reduced to 1 ppm or below when the concentration of the iodine compounds in the crude acetic acid to be treated is high.
To remove iodine compounds from crude acetic anhydride or a mixture of crude acetic anhydride and crude acetic acid, the process described in the above U.S. Pat. No. 4,039,395 wherein methanol is introduced into a distillation column and the process described in the above Japanese Patent Publication-B No. 61-8811 wherein methyl acetate is introduced into a distillation column may be employed. Although these processes are effective in the case of acetic acid alone, it has been found that these processes have the following defects in addition to the above-described defects when also acetic anhydride is contained: i.e., methanol rapidly reacts with acetic anhydride to form methyl acetate, which has such a low reactivity with hydrogen iodide and acetyl iodide in crude acetic anhydride that no sufficient reaction of the methyl acetate with hydrogen iodide and acetyl iodide can be effected by their mere contact during distillation in the distillation column and, therefore, no intended effect can be obtained unless an impractically large amount of methyl acetate is introduced into the distillation column.
Therefore, an object of the present invention is to provide an effective method of removing ionic iodine compounds such as acetyl iodide and hydrogen iodide from crude acetic anhydride or a mixture of crude acetic anhydride with crude acetic acid formed by continuously reacting dimethyl ether and/or methyl acetate, and optionally water and/or methanol, with carbon monoxide alone or both carbon monoxide and hydrogen, in the presence of a rhodium compound as a catalyst, methyl iodide as a cocatalyst and an iodide as a reaction promoter.